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Anyone who has spent any time quoting jobs in the plastics industry knows that little, if any, science is brought into the pro- cess. For most plastic parts, approximately half of the cost is in the raw material and the other half is related to the machine time required to make the part. The mass of the proposed part is relatively easy to determine with current technology, based on the volume and the solid-state density of the specified material. The number of cavities that will be tooled can be speci- fied so that everyone involved in the quoting process is working from the same general parameters. The big variable then becomes the estimated cycle time. If the process involves a new product, the quoting is done from a two-dimen- sional drawing or a 3D model. But even in a situation where an actual part is available, you can provide this part to five different people in the industry and get five different estimates of the cycle time. Cycle Time: Science vs. Rules of Thumb If you inquire of the participants in this exercise as to how they arrived at the cycle time, you will get a lot of answers that fall into the category of rules of thumb. Factors such as wall thickness, flow length, and specified tolerances will likely be mentioned. But the quantitative relationship between these factors and the estimated cycle time will be elusive. Utilities that are used to determine cycle time can be found everywhere, and it can be a fascinating exercise to examine the underlying assumptions that go into them. A lot of thought has gone into understanding how to calculate cycle time, and some models have been developed that use impres- sive-looking equations. These expressions include some quantities such as wall thickness, the thermal diffusivity of the material, and the temperature of the melt and the mold. But there is always one factor that is challenging to define and is a favorite candidate for employing rules of thumb. This is the temperature the polymer must reach so that the part can be ejected from the mold. What exactly is this temperature? Some programs use the deflection temperature under load (DTUL), also often referred to as the heat-deflection temperature (HDT) of the material. This is a curious choice because it assumes that there is a scientific relationship between the ejection temperature and the DTUL. But if we inquire about the origin of such a relationship, the picture gets a little murky. Ask most people in the industry about the significance of the DTUL and you will get an answer that bears very little resemblance to the true picture. A reading of the ASTM or ISO method used to measure the DTUL will show that it is the temperature at which a certain deflection is obtained while placing a sample of a very specific and regular geometry under a constant stress. A flex bar is mounted on a three-point bending fixture and the specified stress is applied to the center of the bar. The sample is What temperature must the polymer reach so the part can be ejected from the mold? Here, more than for any other variable, 'rules of thumb' unfortunately prevail. By Mike Sepe PART 1 Part ejection temperature is often assumed to be related to HDT or DTUL—the temperature at which a material achieves a certain modulus—but is it? Shown here: a flex modulus tester. (Photo: Teknor Apex). 30 JUNE 2017 Plastics Technology PTonline.com K now How MATERIALS